A SOFC comprises an oxygen-ion conducting electrolyte, a cathode where oxygen is reduced and an anode where hydrogen is oxidised. The overall reaction in a SOFC is that hydrogen and oxygen electrochemically react to produce electricity, heat and water. The operating temperature for a SOFC is in the range 600 to 1000° C., often 650 to 1000° C., more often 750 to 850° C. A SOFC delivers in normal operation a voltage of normally below about 0.75V. The fuel cells are therefore assembled in stacks in which the fuel cells are electrically connected via interconnector plates.
Typically, such fuel cells are composed of Y-stabilized zirconia (YSZ) electrolyte together with cathode and anode electrodes and contact layers to the electron conducting interconnect plate. The interconnect makes the series connection between the cells and is normally provided with gas supply channels for the fuel cell. Gas-tight sealants are also usually provided to avoid the mixing of air from the cathode region and fuel from the anode region and they provide also for the proper bonding of the fuel cell units with the interconnector plates. The sealants are thus vitally important for the performance, durability and safe operation of the fuel cell stacks.
During operation the SOFC is subjected to thermal cycling and may thereby be exposed to tensile stress. If the tensile stress exceeds the tensile strength of the fuel cell, it will crack and the whole fuel cell stack will malfunction. One source of tensile stress in the SOFC arises from the discrepancies between the thermal expansion coefficients (TEC) of the cell stack components. The high operating temperature and thermal cycling of a SOFC stack require that the interconnect plates are made of materials which have a TEC similar to that of the fuel cell units. It is today possible to find suitable materials for interconnect plates which have substantially the same TEC as the cells.
Another source of tensile stress which is more difficult to avoid results from the discrepancy in TEC of the sealant, often a glass sealant, with respect to the interconnect plates and the cells in the fuel cell stack. It is normally recognized that the thermal expansion coefficient (TEC) of the sealant should be in the range 11-13·10−6K−1 (25-900° C.), thus corresponding to the TEC of the interconnector plate and/or the fuel cell in order eliminate cracks formation in the fuel cell components. Furthermore, the sealing material has to be stable over a time span of say 40,000 h without reacting with the other materials and/or ambient gasses.
A common material used in gas-tight sealants is glass of varying compositions and much work has been concentrated on development of suitable glass compositions:
Our EP-A-1,010,675 describes a number of glass sealing materials suitable for SOFC, including alkaline oxide silicate glasses, mica glass ceramics, alkaline-earth oxide borosilicate/silicaborate glasses and alkaline-earth alumina silicates. This citation teaches the preparation of a glass sealing material based on dried glass powder and a filler material. The TEC of the glass powder may be as low as 7.5·10−6 K−1 and accordingly, filler material is added to increase the TEC in the final glass powder so that it substantially matches that of the interconnector plates and fuel cell units having TEC of 9-13·10−6K−1.
EP-A-1,200,371 describes a glass-ceramic composition which is provided as a blend of Al2O3, BaO, CaO, SrO, B2O3 and SiO2 within specific ranges. The glass and crystallized (after heat treatment) glass-ceramic show TEC ranging from 7·10−6 K−1 to 13·10−6 K−1. However, a considerable amount of BaO is required in the glass ceramic composition to obtain the high TEC. Prior to heat treatment, the TEC of the glass-ceramic substantially matches that of the other solid ceramic components (within 30%).
S. Taniguchi et al. Journal of Power Sources 90 (2000) 163-169 describes the use of a silica/alumina (52 wt % SiO2, 48 wt % Al2O3; FIBERFRAX® FFX paper #300, Toshiba Monofrax, thickness 0.35 mm) ceramic fiber as sealing material in solid oxide fuel cells. This sealant is able to suppress electrolyte-cracks in the fuel cell but the sealant properties are insufficient, as gas leakage is detected near the sealing material.
US-A-2003/0203267 discloses the use of multilayer seals including the use of a glass material containing 58% SiO2, about 9% B2O3, about 11% Na2O, about 6% Al2O3, about 4% BaO, and ZnO, CaO and K2O.
It is an object of the present invention to provide a solid oxide fuel cell stack containing a gas-tight sealant which does not initiate cracking in the cells and which has low reactivity with other cell stack components.
It is another object of the invention to provide a solid oxide fuel cell stack containing a gas-tight sealant which enables faster production of the stacks with better thickness tolerance of the sealant across the stack.
It is yet another object of the invention to provide a solid oxide fuel cell stack containing a gas-tight sealant which enables low electrical conductivity at the operation temperature of the stack.
These and other objects are solved by the invention.